The millimeter wave spectra of isocyanic and isothiocyanic acids

Abstract

An investigation of the mm-wave rotational spectra of the very slightly asymmetric tops HNCS, DNCS, HNCO and DNCO has been carried out in the frequency range 80 000–190 000 Mc/sec. Several R-branch, a-type, transitions were observed for each molecule and the rotational constants A 0, B 0, and C 0 were determined for the four species containing the most abundant isotopes of N, C, O, or S. B 0 and C 0 were also found for HNCS 34. The centrifugal distortion constant D J was obtained for each abundant form. The constants are HNCS: A 0 = 1 483 000 ± 167 000 Mc/sec, B 0 = 5883.42 ± 0.02 Mc/sec, C 0 = 5845.62 ± 0.02 Mc/sec, D j = 1.17 ± 0.05 Kc/sec; DNCS: A 0 = 723 400 ± 25 000 Mc/sec, B 0 = 5500.51 ± 0.05 Mc/sec, C 0 = 5445.26 ± 0.05 Mc/sec, D j = 1.22 ± 0.31 Kc/sec; HNCS 34: B 0 = 5744.81 ± 0.20 Mc/sec, C 0 = 5708.73 ± 0.20 Mc/sec; HNCO: A 0 = 956 400 ± 50 000 Mc/sec, B 0 = 11 071.02 ± 0.05 Mc/sec, C 0 = 10 910.58 ± 0.05 Mc/sec, D j = 3.5 ± 0.5 Kc/sec; DNCO: A 0 = 534 500 ± 70 000 Mc/sec, B 0 = 10 313.61 ± 0.05 Mc/sec, C 0 = 10 079.67 ± 0.05 Mc/sec, D j = 2.9 ± 0.5 Kc/sec. The centrifugal distortion K-pattern is different in character for each of the four molecules. Agreement between calculation and experiment is obtained for HNCO and DNCO with the additional constants; HNCO; D JK = +837 Kc/sec, H JJK = −0.056 Kc/sec, H JKK = +2.81 Kc/sec. DNCO: D JK = −227.1 Kc/sec, H JJK = +0.15 Kc/sec, H JKK = −5.61 Kc/sec, H JJJ = 0, utilizing the expression given by Costain. This expression gives a poor fit for DNCS and no fit for HNCS, and at present no treatment has been found to fit or explain satisfactorily the K-patterns for these two molecules. An improved molecular structure has been calculated for HNCS the structural parameters being revised to r(HN) = 0.988 7 ± 0.003 A, r(NC) = 1.216 4 ± 0.007 A, r(CS) = 1.560 5 ± 0.003 A < HNC = 134°59′ ± 10′. A number of lines due to rotational transitions of vibrationally excited molecules were also observed and are assigned to the states v 5 = 1 and v 6 = 1 for HNCS and HNCO and probably, in addition, v 4 = 1 for DNCS.